Levels of estrogen may be the key to this association: High levels of estrogen are associated with an increased risk of breast cancer, but women who suffer migraines appear to have low levels of the hormone, which may account for the reduction in breast cancer risk, the researchers explained.

"Women who suffer migraines suffer them either when they have fluctuations in their hormone levels or particularly low levels of hormones," said lead research epidemiologist Dr. Christopher I. Li.

Li noted that during pregnancy, when estrogen levels are high, migraines often stop. "Among migraine sufferers who reach the third trimester, 80 percent of them will not suffer any migraines," he said.

The report was published in the November issue of Cancer Epidemiology, Biomarkers & Prevention.

"Migraine brains are more sensitive to many exogenous and endogenous factors, of which falling estrogen levels are an important one for many female migraineurs," Drexler said. "However, female migraineurs are not known to have consistently lower levels of estrogen than are non-migraineurs."

It is not clear that the lower incidence of breast cancer in women with migraines proves that the reason is lower estrogen levels, Drexler said.

"Genetic factors may also be playing a role, as well as exogenous factors such as medication use, smoking and alcohol use, which may vary between migraineurs and non-migraineurs," Drexler said. "In summary, an interesting study, but more work needs to be done to conclude that it has furthered our understanding of the biology of migraine or of breast cancer."

For the study, Li's team collected data on 3,412 postmenopausal women. Among these women, 1,938 had been diagnosed with breast cancer, and 1,474 had no history of the disease. The researcher also gathered information about migraines from the women.

The researchers found that women who suffered from migraines had a 30 percent lower risk of developing breast cancer compared with women with no history of migraine.

Li noted that more work needs to be done to nail down the reason for the apparent protective effect of migraine for breast cancer. "Advancing our understanding of the mechanisms of migraine may improve our understanding of how we could potentially reduce breast cancer risk," he said.

Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, thinks the finding is interesting, but has no clinical implications.

"There is a decreased risk for women with migraines to develop breast cancer," Lichtenfeld said. "But in practical implications -- what should a woman do differently -- there is no action a woman or her health-care professional would take as a result of this report."

Migraine expert Dr. Stephen Silberstein, director of the Jefferson Headache Center at Thomas Jefferson University Hospital in Philadelphia, said the findings are flawed, because using self-reported migraine data is not sufficient to determine whether the women actually suffered migraine or not.

"This study doesn't prove anything," Silberstein said. "It's not that I don't believe the results, it's that the results are not believable."

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